why were the pigs immediately accepted as leaders?

There are a few reasons the pigs were accepted as leaders. The visionary of the entire revolution was a pig, Old Major, who outlines his dream of a world free of humans at the very beginning of the novel. After Old Major's death, the pigs, who are described as "the cleverest" of the animals, organize Old Major's thoughts into a coherent belief system they call "animalism". The three most important organizers are Napoleon, Snowball and Squealer. Snowball is the one that paints the seven commandment on the wall. Thus, by taking leadership early, the rest of the animals, who are not used to being leaders themselves, are easy targets for the pigs who seem clever and have the other animals' interests at heart.